I think 65 nm Brisbane core is better to oc, because the lower voltage, actually I bought AMD 4000+ X2 from newegg few day ago. I oc the 4000+ 2.1 ghz to 3.11 but now I'm slow to 3.0 ghz to run Prime95, no freeze, no crash everything is fine, but is important the motheboard and the memory, because if not limitate your oc. Ahhh!!!! good cooler if mandatory I use Scynthe Ninja is like monster but is better to make the job. Another reason or positive is multiplier are half number (9.5, 10.5, 11.5) give slighty more oc if compare with full number of Windsor (9, 10, 11). Everything here is about the heat, lower voltage and 65 nm lower tempeture.
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AMD X2 4000+ Brisbane @3.0 ghz
Biostar T-force 550
2 gb G.Skill DDR2 800 Ram
160 SATA II
ATI 1900XTX 512 mb